Equation (REF ) includes the parametrizations used in several previous calculations.
In the original work of Ref. [1]} the authors used \(\xi _0=1\)  and \(\xi _9\in (-1,\infty )\)  with all other parameters set to zero.
The chosen value of \(\xi _9\)  allows one to consider either a slighty prolate momentum distribution (\(-1<\xi _9<0\) ), which is elongated along the beam axis, or an arbitrarily oblate distribution (\(\xi _9>0\) ), which is squeezed in the direction of the beam axis.
In the calculations of Ref. [2]}, [3]}, where ellipsoidal asymmetry was included for the first time,
the authors used \(\xi _0=1\)  and various values of \(\xi _2\)  and \(\xi _9\)  with all other parameters zero.
In Ref. [4]} the authors considered a one parameter deformation using \(\xi _0=1+\sigma \) 
and \(\xi _9=-\sigma \)  with \(\sigma \in (-1,\infty )\) . The resulting distribution is slightly oblate for \(-1<\sigma <0\)  and prolate to any degree for \(\sigma >0\) .
